UPDATE FROM is not working with multiple update of same row - sql

Question

I have #Products and #TempProducts tables. I want to Update #Products from #TempProducts.
My code:
DECLARE #Products TABLE(
Id INT,
Name NVARCHAR(255),
Description NVARCHAR(255));
DECLARE #TempProducts TABLE(
RowNumber int,
Id INT,
Name NVARCHAR(255),
Description NVARCHAR(255));
INSERT INTO #Products(Id,Name,Description) VALUES(1,'Name1','Desc1');
INSERT INTO #TempProducts(RowNumber,Id,Name,Description)
VALUES(1,1,'NewName1',NULL);
INSERT INTO #TempProducts(RowNumber,Id,Name,Description)
VALUES(2,1,NULL,'NewDesc1');
INSERT INTO #TempProducts(RowNumber,Id,Name,Description)
VALUES(3,1,NULL,'NewDesc2');
INSERT INTO #TempProducts(RowNumber,Id,Name,Description)
VALUES(4,1,'NewName2',NULL);
WITH TP AS
(
SELECT TOP (100000) RowNumber,Id,Name,DESCRIPTION
FROM #TempProducts
ORDER BY RowNumber ASC
)
UPDATE P
SET Name = ISNULL(TP.Name,P.Name),
Description = ISNULL(TP.Description,P.Description)
FROM #Products P
INNER JOIN TP
ON P.Id = TP.Id
SELECT * FROM #Products
Expected:
Id Name Description
---------------------------
1 NewName2 NewDesc2
But getting:
Id Name Description
---------------------------
1 NewName1 NewDesc1
Note that I am ordering by ORDER BY RowNumber means Update RowNumber1 first then second and so on.

Answer 1

If there are multiple matches, then SQL Server does not specify which gets updated. If you want the last (or first) value updated, then use window functions to find it:
WITH TP AS (
SELECT t.*,
ROW_NUMBER() OVER (PARRTITION BY ID ORDER BY RowNumber DESC) as seqnum
FROM (SELECT TOP (100000) tp.*
FROM #TempProducts ORDER BY RowNumber ASC
) t
)
UPDATE P
SET Name = ISNULL(TP.Name,P.Name),
Description = ISNULL(TP.Description,P.Description)
FROM #Products P INNER JOIN
TP
ON P.Id = TP.Id AND seqnum = 1;

Answer 2

Use caution when specifying the FROM clause to provide the criteria
for the update operation. The results of an UPDATE statement are
undefined if the statement includes a FROM clause that is not
specified in such a way that only one value is available for each
column occurrence that is updated, that is if the UPDATE statement is
not deterministic.
I would do it like this:
UPDATE P SET Name = ISNULL(ca1.Name, P.Name), Description = ISNULL(ca2.Description, P.Description)
FROM #Products P
OUTER APPLY(SELECT TOP 1 Name FROM #TempProducts WHERE ID = p.ID AND Name IS NOT NULL ORDER BY RowNumber DESC) ca1
OUTER APPLY(SELECT TOP 1 Description FROM #TempProducts WHERE ID = p.ID AND Description IS NOT NULL ORDER BY RowNumber DESC) ca2